Conventional capillary writers are usually comprised of a tubular sheath or body, an ink reservoir, and a capillary channel which extends to a tubular writing element. In contrast to caps for conventional writing instruments such as pencils, ball point pens, and fountain pens, which require seals which seal only the general area of the writing instrument from which the tip protrudes, capillary writing instrument caps, in addition to performing that sealing function, must also perform a second sealing function of sealing the tubular element or tip to prevent loss of ink or drying of ink. In order to achieve this dual sealing effect, the tip of the writing instrument must be pushed against the tip sealing material with adequate pressure for the tip sealing material to elastically deform. The tip then rests tightly against the tip sealing material resulting in the necessary sealing effect. In known caps for capillary writers the pressure applied by the tip to the tip sealing material frequently results in penetration of the tip into the tip sealing material. This often results in damage to the tip sealing material which impairs or destroys the sealing effect.
In the case of tubular pens having extremely fine points or tips, e.g., diameters of about 0.1 mm, the tubular writing elements are very thin and can be bent easily by the pressure which results from pressing the tubular writing element against the tip sealing material.
Caps or covers for capillary type writing instruments are well known. German patent 1915566 A1, European Patent 0 432 633 A1, German Patent DE 3702785 A1, German Patent 27 56 505, U.S. Pat. No. 4,086,011, Japanese Patent 1-20149, Japanese Patent 61-11016, German Patent 28 49 939, Japanese Patent 1-8391, and Japanese Patent 54-29850 disclose covers for capillary marking pens.
Other disadvantages of known capillary writing instrument caps is that they are complex and expensive to manufacture, leaks can occur at the air compensation seal because of the hardness of the materials (often greater than 70 Shore D) employed at this seal, and often a separate spring means may be necessary to bring the soft tip sealing material into contact with the tubular writing element.